In the last few years lasers emitting pulses in sub-nanosecond range entered the dermatology market. Thermal relaxation time of a skin particle is proportional to its size. The sub-ns pulse disrupts a small particle whose thermal relaxation time is in the sub-nanosecond region easier than a pulse with duration of several ns. However, its maximum energy being on the order of several hundreds of mJ is smaller than in the nanosecond region, where it easily enters the J range. Larger particles can be therefore better disrupted with a nanosecond pulse. The optimal laser device intended for skin particles removal should be thus capable to operate with both pulse durations.
A nanosecond pulse with energy in the Joule range can be extracted directly from a laser oscillator for example by a standard Q-switching technique. The origin of the laser oscillation is spontaneously emitted photons within the gain medium. During the energy build up phase the laser oscillation (prelase) is prevented by the Q-switch element. After the Q-switch element is switched to the open state the laser oscillation between the back and forth resonator mirror is allowed and the laser device emits a Q-switched laser pulse. It is outcoupled through a partially transmissive mirror.
Sub-nanosecond pulses can be realized by different techniques. A first option is to use a short laser cavity, usually comprised of two diffusion bonded crystals, where one crystal serves as a gain medium and the other as a passive Q-switch. The faces of the rod are dielectrically coated with reflective layers serving as resonator mirrors. Due to the small volume of the active medium the pulse energy extractable from a microchip cavity is small, usually <1 mJ.
Another approach is to implement a pulse suppression technique. One option is to direct the laser pulse into a cell containing gas with non-linear susceptibility tensor hi. If the pulse is squeezed enough it is scattered in the backward direction. During the stimulated Brillouin scattering the pulse width is suppressed. Pulse width suppression can be achieved also by a fast Pockels cell (EO modulator) inserted between two crossed polarizers.
Regardless of the used technique for the sub-nanosecond pulse generation usually an amplification of its energy is required. Amplification toward several hundred mJ can be achieved in a power amplifier. Arrangement of a master oscillator providing the initial low power pulse characteristics and the power amplifier is abbreviated with MOPA (Master Oscillator Power Amplifier). In a high gain system a sufficient amplification may be achieved by single or double pass of the pulse through the amplifying medium. However, if the gain of the amplifying medium or the seed energy is small, multiple passes are needed. Multiple passes can be achieved in regenerative amplifiers or in special MOPA arrangements.
A double pass amplifier, also called double pass MOPA, usually includes a coupling polarizer, a gain medium, a lambda/4 waveplate and a back reflection mirror. The polarizer reflects the pulse into the amplifier. It passes the amplifying medium followed by the lambda/4 waveplate, and after reflection on the back mirror it passes once again the lambda/4 waveplate and the amplifying medium. A double pass of the lambda/4 waveplate rotates the initial polarization of the pulse by 90 degrees and the polarizer outcouples the amplified pulse.